1. Field of the Invention
The present invention relates to a variable reluctance type angle detector for detecting a rotation angle of a motor or the like.
2. Description of the Related Art
A conventional variable reluctance type angle detector includes a stator in which an exciting winding and n-phase output windings are provided in slots and an iron core having a shape in which a gap permeance between the iron core and the stator is sinusoidally changed corresponding to an angle θ.
In order to form the exciting winding, all exciting coils in which wires are wound in a winding direction which is alternately changed to a normal direction or a reverse direction in each of the slots, are connected in series such that the number of poles of the exciting winding is equal to the number of teeth (the number of slots) of the stator.
In order to form the output windings, output coils in which wires are wound are connected in series such that a voltage induced in the output winding of one phase is sinusoidally distributed.
When a sinusoidal voltage is applied to the exciting coils in the above-mentioned structure, a sinusoidal current flows into the exciting coils, thereby generating magnetic fluxes in the teeth.
A magnetic flux density of each of the teeth is determined based on a gap permeance between each of teeth of the stator and a rotor. Therefore, an output voltage is produced in each of the output windings as a composite value of magnetic fluxes which cross the output coils wound on the respective teeth.
The gap permeance is changed according to a rotational position of the rotor, so an amplitude of the output voltage changes. Therefore, the angle of the rotor can be obtained from the amplitude of the output voltage (see, for example, JP 3182493 B).
For example, when the conventional variable reluctance type angle detector is attached to a motor driven by an inverter, a noise caused by switching of the inverter is induced to an output winding of the conventional variable reluctance type angle detector, thereby deteriorating the detection precision of the rotational angle.
In order to reduce the noise, it is expected that an exciting ampere-turn is increased to increase an amplitude of a signal component from the output winding. However, a power source is shared with another device, so an exciting power source voltage is limited, with the result that the exciting ampere-turn cannot be freely increased.
Even when an exciting current can be increased, a current density of an exciting coil increases, so that the exciting coil is likely to overheat.